You can see what the founders say in their own words here: http://longnow.org/about/
But their basic goal as I see it is: "encourag[ing] the long view and the taking of long-term responsibility, where 'long-term' is measured at least in centuries".
The disk itself is an example of that. The name obviously hearkens back to the Rosetta Stone, which unlocked the translation of Egyptian hieroglyphics for us: https://en.wikipedia.org/wiki/Rosetta_Stone
With a copy of the Rosetta Disk, as long as some future finder understands any one language spoken today they'll be able to translate any other materials they find from our era.
Will it survive? Will it be useful? Will it even be necessary? More broadly, what does the future mean to us, and what effect do we want to have on it?
Those are all good questions. And my take is that the real purpose of the Long Now is not to answer them, but to get people asking them. So good work, your post helps fulfill the mission.
The question I am yet to find a good answer for, with regards to the Long Now proposal, is this: why?
Like, if you know you're going to have a situation where this immensely important information is going to have to be re-discovered, some distant time into the future, because .. after all, entropy is the natural state of things and who could expect we'll still speak English/C++ 100,000 short years from now .. so we'll teach them again, some day way in the future. But, why? Is it merely to fight the entropy, and say: no matter the light, here is the shadow we cast today, and millions of days, millions of years into the future?
Because that is some mighty high pretense.
However, perhaps you have a better answer, as a more coherent observer of the Long Now technological/social infrastructure? Is it really just to challenge time in a technological sense, or .. perhaps .. is this one area where secularism is becoming what it resists, and indeed such foundations are a new religion/cult/etc.?